Fitbits and other wrist-worn fitness devices promise to keep track of your heart rate. But, new research suggests they are less accurate than thought during certain exercises.
“If you need to know your heart rate with accuracy when exercising — either because you are training for a marathon or have safe heart rate limits set by your doctor– wrist-worn monitors are less accurate than the standard chest strap,” said study author Dr. Marc Gillinov, a heart valve research, thoracic and cardiovascular surgery expert at the Cleveland Clinic.
The study compared heart rate reading on wrist-worn devices to readings from a continuous 4-lead electrocardiogram (EKG) and a chest strap monitor. Like an EKG, the chest strap measures the electrical activity of the heart.
Depending on the type of activity, the wrist devices were up to 34 beats a minute off. The wrist trackers could either overestimate or underestimate heart rate, Gillinov said.
The study included 50 volunteers. Their average age was 38. They tested popular wrist-worn fitness trackers, including the Apple Watch, Fitbit Blaze, Garmin Forerunner 235, and TomTom Spark Cardio.
The studt recorded volunteers’ heart rates at rest and after light, moderate and vigorous exercise on a treadmill, stationary bike and elliptical trainer. All of them exercised for 18 minutes.
The chest strap monitor closely matched the readings from the EKG, which is the gold standard for measuring heart rate.
And the wrist-worn devices were fairly accurate when a person was at rest.
Most wrist devices gave acceptable readings during treadmill activity. However, they were fairly inaccurate while bicycling or using the elliptical, the study revealed. Fitbit’s maker took issue with the findings.
“We stand behind our heart-tracking technology. Fitbit trackers are not intended to be medical devices,” Fitbit said in a statement. “Unlike chest straps, wrist-based trackers fit conveniently and comfortably into everyday life, providing continuous heart rate for up to several days without recharging [the device’s batteries].”
The San Francisco-based company added that internal studies involving 60 volunteers showed the device has an average error of 6 percent or less for measuring a person’s heartbeat.
Of all the wrist devices tested, the Apple watch seemed to fare the best. It performed well during bicycling and on elliptical machines without arm levels.
Why might there be inaccuracies?
Wrist-worn devices use optical sensing, or light, to measure blood flow, the researchers said.
“It’s not measuring what the heart does, but rather blood flow — basically the volume of blood in the tissue,” Gillinov explained.
Wrist-worn devices also introduce more variables that can result in incorrect readings, he said. Insufficient contact with the skin due to sweating, poor fit or skin color are a few examples.
“Even though all these wrist-worn monitors work by the same general principles, there is considerable variation among them,” he said.
“Overall, they were most accurate when someone was using the treadmill at low intensity and worst when exercising on the elliptical at high intensity,” Gillinov added.
The study is to be presented at the upcoming annual meeting of the American College of Cardiology, in Washington, D.C. Findings presented at meetings are typically viewed as preliminary until they’ve been published in a peer-reviewed journal.
The American Heart Association explains target heart rates.
SOURCE: American College of Cardiology, news release, March 8, 2017